3-23PULSE DEMODULATIONPulse modulation is used in radar circuits as well as communications circuits, as discussed in chapter2. A pulse-modulated signal in radar may be detected by a simple circuit that detects the presence of rfenergy. Circuits that are capable of this were covered in this chapter in the cw detection discussion;therefore, the information will not be repeated here. A RADAR DETECTOR, in its simplest form, mustbe capable of producing an output when rf energy (reflected from a target) is present at its input.In COMMUNICATIONS PULSE DETECTORS the modulated waveform must be restored to itsoriginal form. In this chapter you will study three basic methods of pulse demodulation: PEAK,LOW-PASS FILTER, and CONVERSION.PEAK DETECTIONPeak detection uses the amplitude of a pulse-amplitude modulated (pam) signal or the duration of apulse-duration modulated (pdm) signal to charge a holding capacitor and restore the original waveform.This demodulated waveform will contain some distortion because the output wave is not a pure sinewave. However, this distortion is not serious enough to prevent the use of peak detection.Pulse-Amplitude DemodulationPeak detection is used to detect pam. Figure 3-21 includes a simplified circuit [view (A)] for thisdemodulator and its waveforms [views (B) and (C)]. CR1 is the input diode which allows capacitor C1 tocharge to the peak value of the pam input pulse. Pam input pulses are shown in view (B). CR1 is reversebiased between input pulses to isolate the detector circuit from the input. CR2 and CR3 are biased so thatthey are normally nonconducting. The discharge path for the capacitor is through the resistor (R1). Thesecomponents are chosen so that their time constant is at least 10 times the interpulse period (time betweenpulses). This maintains the charge on C1 between pulses by allowing only a small discharge before thenext pulse is applied. The capacitor is discharged just prior to each input pulse to allow the output voltageto follow the peak value of the input pulses. This discharge is through CR2 and CR3. These diodes areturned on by a negative pulse from a source that is time-synchronous with the timing-pulse train at thetransmitter. Diode CR3 ensures that the output voltage is near 0 during this discharge period. View (C)shows the output wave shape from this circuit. The peaks of the output signal follow very closely theoriginal modulating wave, as shown by the dotted line. With additional filtering this stepped waveformclosely approximates its original shape.Figure 3-21A.Peak detector. CIRCUIT OF PEAK DETECTOR.